Investigating oxygen reduction intermediates on iron macrocycles Silvia Favero 1 , Alexander Bagger 1 , Ruixuan Chen 1 , Reshma Rao 2 , Ifan E. L. Stephens 2 , Maria-Magdalena Titirici 1 1 Department of chemical engineering, Imperial College London, UK, 2 Department of materials, Imperial College London, UK Pyrolized iron-based catalysts are one of the most promising alternatives to platinum, for the oxygen reduction reaction (ORR) taking place at the cathode of fuel cells. However, the pyrolysis step leads to the formation of a plethora of iron sites, which complicates fundamental studies of the catalyst and challenges the characterization of the active sites. In turn, this hinders the rational optimization of pyrolized Fe-based catalysts. On the other side, iron macrocycles, with their well-defined structures, offer a platform for the study of the oxygen reduction reaction on FeN 4 active sites. These macrocycles exhibit one or two distinctive peaks in their cyclic voltammograms, the origin of which is still debated. 1-3 There have also been reports of the position of the high- potential peak correlating to the activity of the motif, 4 making it a powerful performance descriptor and a tool to draw structure-property relationships of FeN 4 sites. Hereby, several iron macrocycles with different ligands and active sites were investigated, using electrochemistry, in-situ characterization techniques and DFT simulations. In-situ UV-Vis results (shown in Figure 1 for the molecule FePC) show potential-dependent spectral changes in concomitance with the CV peaks. This, with the help DFT simulations, allowed us to identify of the origin of the spectral change and to propose the existence of two families of macrocycles, with drastically different ORR activity. One group is characterized by two CV peaks, high ORR activity and low peroxide yield, while the other features a single CV peak, poor catalytic performance and higher peroxide production. This study provides a first step in understanding what controls the activity of FeN4 sites and offers a tool to unravel fundamental properties of pyrolized FeN x catalysts.
References 1. Aludairi, A. et al. Resolving the Iron Phthalocyanine Redox Transitions for ORR Catalysis in Aqueous Media. J. Phys. Chem. Lett. 8 , 2881–2886 (2017). 2. Ramaswamy, N., Tylus, U., Jia, Q. & Mukerjee, S. Activity descriptor identification for oxygen reduction on nonprecious electrocatalysts: Linking surface science to coordination chemistry. J. Am. Chem. Soc. 135 , 15443–15449 (2013). 3. Zagal, J. H. & Koper, M. T. M. Reactivity Descriptors for the Activity of Molecular MN4 Catalysts for the Oxygen Reduction Reaction. Angew. Chemie - Int. Ed. 55 , 14510–14521 (2016). 4. Zagal, J. H. et al. Towards a unified way of comparing the electrocatalytic activity MN4 macrocyclic metal catalysts for O2 reduction on the basis of the reversible potential of the reaction. Electrochem. commun. 41 , 24–26 (2014).
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